Long term physical ageing of polycarbonate at room temperature: dynamic mechanical measurements

The loss peak, called alpha', appearing as a shoulder in the left wing of the a! peak in dynamic mechanical measurements is used to follow the very long term ageing at room temperature of samples of Polycarbonate rejuvenated by cold rolling or by quenching prior to ageing. This peak shifts to higher temperature with increasing ageing time t(a). An approximate linear increase with In t(a) is found with a similar slope of about 8 K/decade for both kinds of samples. The shape and the magnitude of the peak are strongly dependent on the treatment given to the samples before ageing. It is assumed that ageing at room temperature and below results from an activated process additive to the annealing process prevailing at higher temperatures up to T-g. This ageing process which depends of the pretreatment is characterized by a lower activation energy than the annealing one. Taking these assumptions into account, experimental data are shown to agree reasonably with the predictions of a model previously proposed. (C) 1999 Kluwer Academic Publishers.